Field of the Invention
This invention relates to accelerometers and more particularly to a class of thermally insensitive hung mass accelerometers that use differential Eddy current sensing to provide improved sensitivity at lower cost and with higher reliability.
Description of the Related Art
The basic, open-loop accelerometer consists of a proof mass attached to a spring or flexure. The mass is constrained to move only in-line with the spring. Acceleration causes deflection of the mass. The displacement of the mass is measured. The acceleration is derived from the values of displacement, mass, and the spring constant. The system must not be driven at a resonance that would cause it to exceed its travel limits and bottom out. One way to achieve this is to damp the system. Another way is to mount the accelerometer on an isolation system. A closed-loop accelerometer typically achieves higher performance by using a feedback loop to cancel the deflection, thus keeping the mass nearly stationary. Whenever the mass deflects, the feedback loop causes an electric coil to apply an equally negative force on the mass, canceling the motion. Acceleration is derived from the amount of negative force applied. Because the mass barely moves, the sensitivity to non-linearities of the spring and damping system are greatly reduced. In addition, this accelerometer provides for increased bandwidth past the natural frequency of the sensing element. (Excerpted from Wikipedia “Inertial Navigation System” and “Accelerometer”).
Conceptually, an accelerometer behaves as a damped mass on a spring. When the accelerometer experiences acceleration, the mass is displaced to the point that the spring is able to accelerate the mass at the same rate as the accelerometer body. The displacement is then measured to give the acceleration.
The performance of an accelerometer is primarily a combination of its bias stability and scale factor error. Bias stability is the acceleration measured by the device if the actual acceleration is zero. If the device is not accelerating due to imperfections of the device and electronics the readout will be nonzero. The scale factor error reflects the error as proportional to the actual acceleration. If for example the device is accelerating at 1 g (32 feet/sec/sec) [1 g is either 32.2 ft/sec/sec or 9.8 meters/sec/sec] and the device reads out 1.1 g, the scale factor error is 10%.
U.S. Patent Publication US2014/0157897 published Jun. 12, 2014 entitled “Hung Mass Accelerometer with Differential Eddy Current Sensing” and assigned to Raytheon Company introduced a new class of open-loop accelerometer that provides improved performance at lower cost and with higher reliability. As illustrated in FIGS. 5a and 5b and as described in paragraph [0046], an embodiment of an open-loop hung mass accelerometer 100 sans the electronics comprises a single piece of metal 101 (e.g. Ti 6A1-4V or 17-4PH stainless steel) machined to form a body 102, top and bottom flexures 104 and 106 and a proof mass 108 suspended between the flexures inside an internal cavity 109 to deflect along an axis 110 through the center of the body. Each flexure includes three flexure legs that are spaced at 120°. Each flexure leg is attached in the axial direction between the proof mass 108 and the body 102. Eddy current sensor heads 112 and 114 extend through holes in the body and through the flexures along axis 110. To position the sensor heads close to the proof mass, the sensor heads comprise a hollow stainless steel cylinder typically 0.25 to 0.5 inches long. At the end of the cylinder is a coil of copper wire that does the sensing/measurement. Between the copper coil and the mounting surface (0.25 to 0.5 inches away) are: the steel cylinder, G10 and various epoxies.